Precision potentiometers



United States Patent O PRECISION POTENTIOMETERS Herbert A. Gottschall, Nutley, N. J., S. Frederic Guggenheim, Riverdale, N. Y., and Arnold S. Louis, Teaneck, N. J., assignors to Myron A. Coler, Scarsdale, N. Y.

This invention relates to rectilinear variable resistance devices.

Basically, a rectilinear variable resistance device consists of a straight line resistance element, a movable contact in engagement with the resistance element and a shaft for moving said movable contact in accordance with a condition to be sensed. The practical embodiment of such a device includes, in addition, a case and a bearing passing through the case which supports the shaft, in turn the shaft supports an insulator, commonly termed a brush block" from which is sprung the contact by means of a spring member. Means are provided to make electrical connection from the movable contact to a collector which is connected to a terminal.

There have been many embodiments of such a variable resistance device commercially available but to the best of our knowledge they all have common failings which have not heretofore been overcome.

One of the failings of the prior art devices has been play between the actuating shaft and thebrush block. One prior solution has been the provision of a spring between the brush block and shaft to eliminate this play. However, when the apparatus is employed as a precision potentiometer inaccuracies are noted, because of spring fatigue and deformation. The potentiometer of this invention is not dependent on spring members to prevent end play, as it employs a construction permitting a close tolerance mechanical linkage.

As has been pointed out earlier, in the conventional rectilinear potentiometer, the shaft normally is supported by a single bearing mounted in the case wall, the brush block is supported from the end of the shaft so that it is cantilevered out from the bearing. Since the brush block has affixed to it a spring member intended for the purpose of providing brush pressure against the resistance element, it will be appreciated that the same spring members will tend to push the cantilevered shaft in a direction away from the resistance track. This is an undesired effect because it produces a variation in brush pressure as the length of the cantilevered portion of the shaft varies. Other disadvantages resulting from this mechanical distortion are the resulting inaccuracies, excessive friction and excessive physical wear.

Some prior art devices avoided this problem by providing guide rails for the brush block, which then traversed the resistance element much like a traveling crane. The disadvantages of the latter solution are the increased size and weight requirements and the increased friction. The device of this invention on the other hand solves the problem in a simple and compact manner.

Briefly stated, a spring member and a brush having a low coefiicient of friction are mounted on the side of the brush block opposite the movable contact and are permitted to act on a surface opposite the element. The added spring and brush do not serve to make electrical connection but merely as a counterbalancing means.

The device of this invention is also characterized by ice being compact, easily assembled and free of loose mechanical linkages.

Accordingly, it is an object of this invention to provide an improved rectilinear variable resistance device.

It is another object of this invention to provide a rectilinear variable resistance device having means to prevent end play between a rotatable shaft and brush block.

It is another object of this invention to provide an easily assembled rectilinear variable resistance device.

It is a particular object of this invention to provide a rectilinear variable resistance device having constant brush pressure.

Still another object of this invention is to provide a rectilinear variable resistance device, not subject to deflection of the actuating shaft.

Still a different object of this invention is to provide a. precision potentiometer.

Still other objects and advantages will be pointed out with particularity and still others will become apparent as the following description proceeds taken in consideration with the accompanying drawings wherein in the various figures like numerals refer to like members.

In the drawings:

Figure 1 is an enlarged cross-sectional showing of the potentiometer of this invention.

Figure 2 is a cross-sectional showing taken along line 22 of Figure 1.

Figure 3 is a cross-sectional showing taken along line 33 of Figure 1.

Figure 4 is a schematic diagram showing the internal circuit of the apparatus of this invention.

Referring now to the drawing, there is shown a stainless steel tubular casing 10 into which is force fit a stainless steel end plug 12. Plug 12 is faced by plastic insulator disc 14, which is cemented into place. A resistance track 16, comolded to an insulator plastic base 18 is mounted in casing 10 by means of screws 20. A collector track 22 is provided. Brush 24 is arranged to traverse resistance track 16 in accordance with a condition to be sensed. The voltage sensed by the brush 24 is applied to collector track 22 by means of brush 26 which is connected electrically by beryllium copper springs 28 and 30 and shunt 32.

Lead wires 34 make electrical contact to resistance track 16 and collector track 22 through hollow rivets 36 to which they are soldered in assembly. Jumpers 38, make connection from the resistance track 16 and collector track 22 to separate rivets 36. The leads 34 are potted in epoxy resin 40 to seal them into member 18. The unpotted lead ends are fed through slot 39 which is milled into casing 10.

The brush assembly 41, comprising brushes 24 and 26;

springs 28, 30 and jumper 32, is mounted on brush block 42. Brush block 42 which is formed of a linen filled phenolic plastic is tapped to receive screws 44 which hold the assembly 41 in place.

A feature of this invention is the provision of a counterbalancing spring member 46 and brush 48 which rides on case 10. The spring member 46 is mounted on brush block 42 by means of screws 50. This improvement eliminates the cantilever effect on actuating shaft 52.

Another feature of the invention is the method of mounting the brush block 42 on shaft 52 so as to permit rotation of the shaft while avoiding end play.

Stainless steel shaft 52 is provided with a reduced diameter 54 which passes through a correspondingly sized hole 56 in bushing 61 located in brush block 42.

Stainless steel washers 58 and 60 protect brush block 42. If a freely turning shaft 52 is desired then a bearing material suchas porous bronze may be employed as bushing 61.

Nut 62 is tightened onto threaded portion 64 until end play is eliminated but not so tight as to cause seizure and '3 .prevent.rotation of shaft 52. The nut may be of the selflocking type or may be soldered in place. As an alternative to the solder or self-locking nut an extended threaded .portion :64 :and .a :second locking vnut could be employed.

Rotation of brush block .42 is prevented 'by means of keyway 68 and phenolic strip 70 bolted to the inside of casing by means of screws 72.

Shaft 52 is supported .in bearing 74 which is .of the porous bronze type. In turn bearing 74is held by stainless steel cap member 76 which is tapped to receive screws 78 :(only one is shown) to lock the cap into the case 10. Flange 80 provides accurate positioning.

The shaft 52 is shown .with a threaded end knob 82, as a coupling means, however, other coupling means may be employed.

A circuit diagram corresponding to a structure of this invention is shown in Figure 4. The same numerals are employed in the schematic diagram .as in Figures 1 and 2.

Resistance element 16 is connected to hollow rivets 36 by means of jumpers 38. In turn leads 34 connected to rivets 36 permit connection to .external circuits.

Brush block v42 carried by shaft 52 supports brush assembly 41, comprising springs 28, 30, jumper 32 and contact members 24 and 26 in series connection with resistance element 116 and collector track 22.

,Collector track 22 is connected to a hollow rivet 36 by jumper 38. A lead 34 connected to rivet 36 provides means for making connection to external circuits.

When used as a voltage dividing means, it is customary practice to connectthe source of voltage to opposite ends of resistance element 16 by means of leads 34 and to derive an output signal proportional to the position of the brush of the wiper 24 with respect to resistance element 16 and takeoff 22 to the output circuit by means of an appropriate pair of leads 34.

The term potentiometer and variable resistance device have been used interchangeable herein. In general a variable resistance device may be employed as a potentiometer or as a rheostat.

Current technical lexicon employs the term potentiometer to describe a voltage proportioning device which may have but little current carrying capacity. It is therefore to be understood that the term as used herein covers the device as a potentiometer and a rheostat.

Thus while in accordance with the statutes we have disclosed the best mode presently contemplated for carrying out the invention be it understood that various modifications and changes may be made without departing from the spirit of the invention and that, accordingly, we wish to be limited only by the scope of the appended claims.

What is claimed is:

I. In a variable resistance device including a casing member; a straight line resistance element supported in said casing member; means for making electrical connection from an external circuit to fixed portions of said straight line resistance element; a shaft supported by said casing member and adapted to be actuated rectilinearly parallel to said resistance element; a brush block positioned on said shaft member and adapted to move therewith; and means for making electrical connection from an external circuit to a selected portion of said resistance element including spring loaded contact making means carried by said brush block; the improvement comprising: a fixed surface located within said casing member substantially opposite and parallel to said resistance element; an electrically isolated spring member carried by said brush block and arranged to act against said fixed surface so as to apply to said brush block a force substantially equal and opposite to the force exerted on said brush block by said spring loaded contact making means.

2. In a variable resistance device including a casing member; a straight line resistance element supported in said casing member; means for making electrical connection from an external circuit to fixed portions of said straightline resistance element; a shaft supported by said casing member and adapted to be actuated rectilinearly parallel to said resistance element; a brush block positioned on said shaft member and adapted to move therewith; and means for making electrical connection from an external circuit to a selected portion of said resistance element including a collector member parallel to said resistance element, a first spring loaded brush means in contact with said resistance element and a second spring loaded brush means in contact with said collector member, and means electrically connecting said first and said second brush means, the improvement comprising: a fixed surface portion located within said casing member substantially opposite and parallel to said resistance element; an electrically isolated spring member carried by said brush block and arranged to act against said fixed surface so as to apply to said brush block a force substantially equal and opposite to the force exerted on said brush block by said first and said second spring loaded brush means.

3. In a variable resistance device including a casing member; a straight line resistance element supported in said casing member; means for making electrical connection from an external circuit to fixed portions of said straight line resistance element; a shaft supported by-said casing member and adapted to be actuated rectilinearly parallel to said resistance element; a brush block positioned on said shaft member and adapted to move therewith; and means for making electrical connection from an external circuit to a selected portion of said resistance element including spring loaded contact making means carried by said brush block; the improvement comprising: a fixed surface located within said casing member substantially opposite and parallel to said resistance element; an electrically isolated spring member carried by said brush block; a brush member carried by said spring member and arranged to act on said fixed surface; said spring member being 'tensioned so as to apply to said brush block a force substantially equal and opposite to the force exerted on said brush block by said spring loaded contact making means.

4. In a variable resistance device including a casing member; a straight line resistance element supported in said casing member; means for making electrical connection from an external circuit to fixed portions of said straight line resistance element; a shaft supported by said casing member and adapted to be actuated rectilinearly parallel to said resistance element; a brush block positioned on said shaft member and adapted to move therewith; and means for making electrical connection from an external circuit to a selected portion of said resistance element including a collector member parallel to said resistance element, a first spring loaded brush means in contact with said resistance element and second spring loaded brush means in contact with said collector member, and means electrically connecting said first'and said second brush means; the improvement comprising: a fixed surface located within said casing member substantially opposite and parallel to said resistance element; an electrically isolated spring member carried by said brush block; a brush member carried by said spring member and arranged to act against said fixed surface; said spring member being tensioned so as to apply to said brush block .aforce substantially equal and opposite to the force exerted on said brush block by said first and said second spring loaded brush means.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,977 Lannerd Nov..19, 1940 2,506,482 Batcheller May 2, 1950 2,668,218 Searle Feb. 2, 1954 2,706,230 Bourns et a1 Apr. 12, 1955 

